1. Field of the Invention
The present invention relates to an inorganic electroluminescence device for converting electricity to light by making use of a phenomenon called inorganic electroluminescence and using an inorganic substance as a luminescence material, and also relates to a light emitting device and a light emission method that makes use of the inorganic electroluminescence device.
2. Background Art
Representative examples of self-luminescent devices from which surface emission can be obtained include organic electroluminescence devices (may also be referred to hereinafter as “organic EL devices”) and inorganic electroluminescence devices (may also be referred to hereinafter as “inorganic EL devices”). An organic EL device has problems of having a short life and being weak against high temperature because an electric current is made to flow through an organic substance. Although in contrast, an inorganic EL device has several advantageous characteristics, such as being operable in a wide temperature range, having a long life, etc., and numerous studies have been carried out toward practical implementation, many of these studies concern excited luminescence by an AC power supply. Thus, in order to avoid demerits of an AC-excited EL device, a device with which luminescence can be obtained by DC drive has been strongly desired.
With an inorganic EL device, a sulfide-based phosphor material is frequently used, and the method for driving the device is, in most cases, driving by AC or a bipolar pulse voltage. This inorganic EL device has a structure in which a phosphor layer 23 is formed by forming a thin film of the phosphor material above a glass substrate 21 by a vapor deposition method, sandwiching the phosphor layer 23 from above and below by insulating layers 25, and further sandwiching with a lower transparent electrode 22 and an upper back surface electrode 24 as shown in FIG. 14. A DC current thus does not flow, and luminescence is obtained by applying an AC voltage of approximately 100 Hz to 10 kHz from an AC power supply 26. In a first half-cycle of the applied voltage, electrons are accelerated and collided with luminescent centers to cause luminescence inside the phosphor, and in the subsequent inverted half-cycle, the electrons are accelerated in an opposite direction and collided with the luminescent centers to cause luminescence again. Thus, in luminescence by AC, luminescence is made to occur twice within a single cycle but not in a continuous manner. If the EL phenomenon can be made to occur continuously, it may be possible to improve luminescence efficiency and obtain a stronger luminescence. Thus, in order to obtain steady luminescence, the luminescence device must be driven by a DC power supply and thereby be supplied constantly with electrons. A DC-driven inorganic EL device was first presented as a dispersion type EL device in 1968, and thin film type DC-driven EL devices prepared by vacuum vapor deposition methods came to be studied from the 1970s. Although numerous studies have been conducted on this DC-driven EL device, a practical device has yet to be developed because it is weak in luminescence and short in life. In a basic structure of a conventional, DC-driven EL device, the phosphor is sandwiched directly between the transparent electrode and the back surface metal electrode. The phosphor and the electrodes are put in direct contact because charges from the electrodes must be injected directly into the phosphor to make a DC current flow in the interior of the phosphor. However, this structure has a property of unstable current flow, with a current flowing suddenly at a certain voltage or higher, and this tends to cause breakdown of the device.
Thus, in a conventional DC-driven inorganic EL device, a structural modification such as that shown in FIG. 15 is made (Non-Patent Document 1). The DC-driven inorganic EL device shown in FIG. 15 has an arrangement where a DC power supply 28 is connected to the lower transparent electrode 22 installed on the glass substrate 21 and the upper back surface electrode 24, and a stabilizing layer 27 is inserted between the phosphor layer 23 and the upper back surface electrode 24. By providing the stabilizing layer 27, the current that flows is restricted and stabilization of the device is achieved. However, with this structure, the film thickness must be set to approximately several pm to several dozen μm to appropriately restrict the current. Peeling of the thin film and other problems thus occur during manufacture, and restrictions are placed in regard to the materials that can be used. Forming of a thin film of Ta2O5, SiO2, or other insulator with a thickness of several dozen nm to several μm by vapor deposition and passing of a current through this film have thus been examined (Non-Patent Documents 2 and 3). Although it has thus become possible to prepare a DC EL device that emits light of considerably high luminance, the level of practical use has yet to be reached in regard to device stability and life.
There has also been proposed an arrangement in which a dielectric insulator, having a metal impurity dispersed therein, is introduced as a thin film to achieve stabilization by passing of current through an impurity level (Patent Document 1). With this arrangement, a BaTiO3 dielectric insulator, having yttrium (Y) dispersed and mixed therein, is used as a resistor. However, this arrangement has problems in terms of stability and life.
Meanwhile, there are inventions of inorganic EL devices that make use of the same luminescence principle as an organic EL device (Patent Documents 2 and 3). The devices according to these inventions can be driven by a DC power supply, and with these devices, holes from a positive electrode are injected into the interior of a phosphor layer through a charge transport layer and electrons from a negative electrode are injected through an electron injection layer. Here, a recombination type phosphor is used, and luminescence is obtained by recombination of holes and electrons inside the phosphor layer via an impurity lever in the interior of the phosphor.
In another thin-film EL device, holes are injected into the interior of a phosphor through a P-type semiconductor and electrons are injected through an N-type semiconductor. Luminescence is then obtained by recombination of the holes and electrons inside the phosphor (Patent Document 4). The devices disclosed in Patent Documents 2, 3, and 4 are inorganic EL devices that are driven by DC power supplies and are charge injection type EL devices. Luminescence is obtained by injecting electrons and holes into the interior of the phosphor and making the electrons and holes recombine. A recombination type phosphor is used as the phosphor, and recombination luminescence via the impurity level in the interior of the phosphor is used. However, issues still remain in regard to luminescence efficiency and life.
There has also been proposed a dispersion-type, DC-driven inorganic EL device with which a phosphor, having zinc sulfide as a host material and having a powder of a metal dispersed and mixed therein, is sandwiched by two electrodes (Patent Document 5). The device disclosed in Patent Document 5 is a device in which charges are directly injected from the electrodes into the powder in the phosphor and is basically a dispersion type EL device that has been present from before.    Patent Document 1: Japanese Published Unexamined Patent Application No. H5-74572    Patent Document 2: Japanese Published Unexamined Patent Application No. 2006-4658    Patent Document 3: Japanese Published Unexamined Patent Application No. 2007-123220    Patent Document 4: Japanese Published Unexamined Patent Application No. 2009-224136    Patent Document 5: Japanese Published Unexamined Patent Application No. 2008-7755    Non-Patent Document 1: M. Higton: Digest of 1984 SID International Symposium (1984) 29    Non-Patent Document 2: H. Matsumoto et al.: Jpn. J. Appl. Phys. 17 (1978) 1543    Non-Patent Document 3: J. I. Pankove: J. Lumin. 40&41 (1988) 97